Foundation Site Preparation - Roots

[ONENGINEER]

We are constructing a timber structure building. The foundation soil is sandy gravel with some silts and clay containing very irregular boulders 0.5 m diameter occasionally to 1 m. We have removed the top soil and see some roots (some decayed but most are fresh roots).
The question is what size of root can be tolerated under the foundation. If we want to get rid of all roots we may need to excavate 1.2 m down, which is very costly for the private owner.
I would appreciate a feedback, if possible.

 

[oldestguy]

For what it is worth, in my 60 years of geotechnical experiences, I have never seen a foundation settlement due to any roots that may have been below them. I have see a few cases where a stump was left under a pavement and later a low spot developed under traffic loads. I also have seen a site where stumps and logs were buried under compacted fill and no problems have come to my attention for a commercial garage built on that fill.

If I had to set a root size to remove from under a house foundation site, maybe a diameter of 5 cm and larger might be cause for removal, but not for under a floor slab. Even that is quite fussy, especially if scattered. Earth has a way of arching over weak places, like animal burrows.

However, boulders are another thing, especially if their former location was not well compacted. Under a building corner that condition can present itself later with cracks.

 

[oldestguy]

I wonder of someone there works for Sears, sitting in the Sears Tower, Chicago. When the Madison,
WI east side store was being built in early 60's, they called and wanted a sample of soil that the floor slabs would sit on. We sent it. That contained some plant roots about 1 mm diameter, as well as a few worm holes. That office bound "engineer" complained that the presence of roots would not be tolerated. Somehow we managed to convince that person all would be OK, and of course it was. It goes to show how non-field guys can cause unnecessary delays on jobs.

 

[Ron]

Agree with oldestguy. If the roots constitute a "root mat", then settlement can occur. That can occur when you have an "ancient" topsoil layer of 4 to 6 inches thick. That should be removed. If they are scattered roots of 1/2" dia or less and constitute less than about 5 to 10 percent by weight of the soil in any given volume, then don't worry about them.

Agree also that the boulders are an issue. Several issues exist with boulders....load distribution, unknown voids, etc.

 

[cvg]

of course the answer would somehow be related to the type of foundation you are constructing, which you have not given.

 

[QAMaster]

I agree with oldestguy, roots should not cause an issue provided they are relatively small and the soil is firm and relatively dense. My rule of thumb for good practice... I advise the contractor to remove any organics larger than the diameter of his thumb.